1. Technical Field
The present invention relates to an angular velocity detection apparatus.
2. Related Art
To detect the behavior of a mobile unit, which is moving, with high accuracy so that the detected behavior is used by a navigation system, a safety device, a mobile data collector, or the like, the yaw angle and bank (roll) angle of the mobile unit must be obtained. While a gyro sensor is typically used to obtain the yaw angle, an accurate yaw angle may not be obtained depending on the type of the mobile unit. For example, if the mobile unit is a motorcycle, a gyro sensor is fixed to the body of the motorcycle. Thus, when the motorcycle is banked, the angular velocity detection axis is tilted, thereby failing to accurately obtain the yaw angle of the motorcycle.
In the case of a motorcycle or the like, the roll angle significantly varies when the motorcycle or the like is moving, as described above, and the yaw angle is influenced by the roll angle to a nonnegligible extent. Thus, the yaw angle must be detected with high accuracy. For this reason, a navigation system disclosed in JP-A-2006-126178 (hereafter referred to as a “first related-art example”) is allowed to measure a turn angular velocity even when the roll angle varies, by using two tuning-fork-shaped gyro sensors. Specifically, in the navigation system, the gyro sensors are provided in a lateral direction with respect to a vertical direction when a mobile unit stands erect in a manner that the detection axes of the gyro sensors are symmetrically tilted in mutually reverse directions by an identical angle. When the mobile unit is tilted, the navigation system obtains a signal variation from each gyro sensor and calculates the signal variation to obtain a roll angle. Also, the navigation system obtains a turn angular velocity using the obtained roll angle.
However, with regard to the first related-art example, if the gyro sensors are mounted on a mobile unit in a manner that the gyro sensors are tilted, the gyro sensors are used in a manner that the detection axes of the gyro sensors are tilted from a vertical direction of the mobile unit. This impairs the detection sensitivity of the gyro sensors with respect to a variation in angular velocity. Specifically, as seen in FIG. 9 that shows the relation between the sensitivity and tilt angle of one gyro sensor, the sensitivity of the gyro sensor is reduced as the tilt angle of the detection axis of the sensor is increased. In particular, if the gyro sensor is a tuning-fork-shaped one, the sensor has only one drive arm and one detection arm. Therefore, the detection sensitivity of the sensor is originally low. For this reason, with regard to the first related-art example, it is difficult to set a large angle for the tilt angle between the two gyro sensors in terms of the detection sensitivity.
On the other hand, in order to obtain a highly sensitive tuning-fork-shaped gyro sensor, there is provided a method for extending a drive arm and a detection arm (vibration arm) of a gyro sensor. However, if this method is used, the size of the gyro sensor is increased. Also, in the first related-art example, the angular velocity detection axis of each tuning-fork gyro sensor extends in a direction in which a vibration arm thereof extends; therefore, the height (size) of the navigation system itself is unavoidably increased. Also, a highly-sensitive tuning-fork-shaped gyro sensor is obtained by increasing the gain (increasing the amplification factor of a signal); however, in this case, the noise signal is also amplified. Thus, the signal-to-noise (S/N) ratio deteriorates, resulting in a reduction in detection accuracy.
Also, the first related-art example requires that the gyro sensors have the same sensitivity or be disposed symmetrically when a mobile unit stands erect. Therefore, a difference in sensitivity between the gyro sensors causes an error in turn angular velocity, resulting in a reduction in accuracy of the navigation system. While it is relatively easy to dispose the gyro sensors in an erect manner or horizontally in terms of the structural design of the gyro sensors, some contrivance must be made to a base or the like on which the gyro sensors are to be mounted if the gyro sensors must be disposed symmetrically at a given angle. This results in an increase in manufacturing cost.